1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method which is preferably applicable to a color printer, a color digital copying machine, and a complex machine having an image area identification and adjustment function for identifying image areas and adjusting image data.
2. Related Background Art
In recent years, there have often been cases in which tandem color printers, tandem copying machines, or complex machines are used. Each of these color image forming apparatuses comprises exposure means, developing devices, and photosensitive drums for yellow (Y), magenta (M), cyan (C), and black (BK) colors, and an intermediate transfer belt and a fixing device.
For example, the Y-color exposure means forms an electrostatic latent image on the photosensitive drum on the basis of arbitrary image information. The developing device forms a color toner image with Y-color toner adhering to the electrostatic latent image on the photosensitive drum. The photosensitive drum transfers the toner image to the intermediate transfer belt. Regarding other M, C, and BK colors, the same processing is performed. The color toner image transferred to the intermediate transfer belt is transferred to paper and then fixed by the fixing device. To form an optimum color image using such type of color image forming apparatus, it is necessary to identify a photographic image, a screened halftone image, or a character area on the basis of image data of a document and to perform image processing based on a result of the identification before the image formation.
Japanese Unexamined Patent Publication (Kokai) No. Hei09-172544 (1997) discloses an image processing apparatus for identifying a type of a target image in FIG. 2 on page 2. According to the image processing apparatus, an identification means identifies a type of an image based on image data when identifying a type of image data obtained from a document reader (scanner). The image data is processed according to a result of the image type identified by the identification means. When executing a magnification/reduction process, the processing means modifies an identification operation of the identification means. In other words, an identification parameter of the processing means is variable according to reading conditions of the document reader. This enables a precise identification of the type of the target image according to processing conditions and reading conditions of the target image.
Japanese Unexamined Patent Publication (Kokai) No. 2000-134472 discloses a digital image forming apparatus for forming an electrostatic latent image by converting document information to electric signals and developing the electrostatic latent image in FIG. 2 on page 3. The digital image forming apparatus comprises a threshold modification means, which modifies a threshold for classifying halftone dots in the document into a character part and an image part to a character processing side or to an image processing side when setting the threshold. In other words, the setting is made by modifying the threshold for classifying halftone dots into an image and a character. This increases sharpness of the character part and realizes a smooth and natural copy image in the halftone-dot image part, matching the document or a user's intention.
Japanese Unexamined Patent Publication (Kokai) No. Hei07-030752 (1995) discloses an image area identifying apparatus for identifying a type of an image in FIG. 2 on page 2. The image area identifying apparatus comprises a photographic image area identification means, a screened halftone image area identification means, and a character area identification means, wherein the image area identifying apparatus finds an average density for each block from image data, finds a density difference between the average density obtained for each block and an average density of adjacent blocks, and identifies a photographic image area, a screened halftone image area, or a character area on the basis of the density difference. This enables a character image area to be discriminated from a non-character image area from a mixed image area including a character image, a photographic image, and a screened halftone image.
Japanese Unexamined Patent Publication (Kokai) No. 2001-053975 discloses an image printing method of printing a document in black and white by using a color document in FIG. 1 on page 9. The image printing method comprises identifying a character part, a photographic part, and a screened halftone part by using signals of R-, G-, and B-color components obtained by reading the color document with a color scanner and combining the signals of the respective colors on the basis of a result of the identification, thereby achieving a variety of printing expressions.
The conventional color image forming apparatuses, however, have the following problems when identifying a photographic image, a screened halftone image, or a character area from image data of a chromatic or achromatic document and performing image processing based on a result of the identification.    (1) In the above four literatures of prior art technology, the image processing means is provided with an area identification function. In the present state of the technology, however, it is not easy to check that the area identification processing was correctly performed with the image processing means. Generally, an image identification parameter is often set on an operating screen easy to operate for a user.
To check whether the image identification parameter was correctly set on the operating screen, it is necessary to read a document image and to check, for example, that a character area is not tinged with any color or that a screened halftone image is formed in a screened halftone area, or the like.    (2) As stated above, unless the image identification parameter is appropriate depending on the document, it is necessary to repeat color adjustment. Furthermore, there is the possibility that it takes a lot of time for color adjustment, thereby deteriorating the operationality at the color adjustment.    (3) Differences in scanner characteristics between individual copying machines may cause different results of identification when the same document is read. For example, in a machine having a different modulation transfer function (MTF), colored characters are output instead of black characters or a screened halftone area is not identified as a screened halftone image due to a difference in an identification result of the chromatic area or the screened halftone area. Therefore, a detection characteristic of the screened halftone image or a detection characteristic of chromatic color identification varies with machines and it may lead to image deterioration.